[工学]第三章 ORACLE数据库结构1009.pptVIP

* The Archiver Process All other background processes are optional, depending on the configuration of the database; however, one of them, ARCn, is crucial to recovering a database after the loss of a disk. As online redo log files fill, the Oracle server begins writing to the next online redo log file. The process of switching from one redo log to another is called a log switch. The ARCn process initiates backing up, or archiving, of the filled log group at every log switch. It automatically archives the online redo log before the log can be reused, so that all of the changes made to the database are preserved. This enables the DBA to recover the database to the point of failure, even if a disk drive is damaged. Archiving Redo Log Files One of the important decisions that a DBA has to make is whether to configure the database to operate in ARCHIVELOG or in NOARCHIVELOG mode. NOARCHIVELOG Mode: In NOARCHIVELOG mode, the online redo log files are overwritten each time a log switch occurs. LGWR does not overwrite a redo log group until the checkpoint for that group is complete. This ensures that committed data can be recovered if there is an instance crash. During the instance crash, only the SGA is lost. There is no loss of disks, only memory. For example, an operating system crash causes an instance crash. * Instance SGA PMON Process Monitor (PMON) Cleans up after failed processes by: Rolling back the transaction Releasing locks Releasing other resources PGA area LGWR LGWR负责把重做日志缓冲区中的数据写入到重做日志文件中。 下面情况下LGWR发生操作： 当系统提交事务时； 当重做日志缓冲区的三分之一满时； 当在重做日志缓冲区中记录的数据超过1MB时； 在DBWR准备把数据缓冲区中已经修改的数据写到数据文件时（之前）。 Every 3 seconds Instance SGA DBWn Redo log buffer Log Writer (LGWR) LGWR writes: At commit When one-third full When there is 1 MB of redo Every 3 seconds Before DBWn writes Control files Data files Redo log files LGWR Database CKPT CKPT--检查点进程 该进程用来同步化数据库的文件，把检查点内容写入数据文件头，更新控制文件。 检查点事件： Log_checkpoint_timeout:定义时间间隔。 Log_chec